1. Field of the Invention
The present invention relates to a system for controlling the synchronous motor of an electric vehicle.
2. Description of the Related Art
There have been known synchronous motor Control systems for electric vehicles. The known synchronous motor control systems control energizing currents by way of pulse width modulation to energize a synchronous motor to propel the electric vehicle, i.e., to operate the synchronous motor in propulsive mode, or to regenerate electric energy from the synchronous motor, i.e., to operate the synchronous motor in a regenerative mode, based on a commutation signal which is in phase with the rotation of the synchronous motor.
However, the conventional synchronous motor control systems for electric vehicles produce output commands in timed relationship to the commutation signal that is detected by a commutation sensor, i.e., a rotational speed sensor, when the synchronous motor is in the propulsive or regenerative mode, and hence do not effect any special control process for producing a lead or lag in phase. Therefore, in the event that the rotational speed of the synchronous motor varies, the currents of the synchronous motor vary in phase because counterelectromotive forces are induced across the coils of the synchronous motor and the reactances of the coils change due to a frequency change. When the currents of the synchronous motor vary in phase, the synchronous motor is caused to rotate out of synchronism, and the efficiency thereof is lowered though the synchronous motor can be operated in the propulsive or regenerative mode.
Since the electric vehicles employ a vehicle mounted battery as a source of electric energy to be supplied to the synchronous motor, the lowered efficiency of the synchronous motor results in a reduction in the distance that the electric vehicles can travel after the battery has been fully charged in a single charging cycle. Consequently, the conventional synchronous motor control systems have not been satisfactory for controlling the synchronous motors of electric vehicles.